1. Field of the Invention
The present invention relates to a lens antenna.
2. Description of the Related Art
In recent years, active research and development have been going on for producing various types of ITS (Intelligent Transport Systems) suitable for use in a new generation. As a result, more and more functions have been used to ensure safe travel in an automobile. In particular, an outside environment detection system, which serves as an eye for an automobile, is considered to be the most important among various types of ITS. For example, there has been developed a detection system using an infrared ray or CCD. However, each of the known detection systems has been found unsuitable for use in rainy weather, or too expensive in their production cost.
In view of the above, it is desirable to use, as an outside environment detection system, a radar which employs a millimeter wave (76 GHz). An antenna suitable for receiving such a millimeter wave is a lens antenna.
A conventional lens antenna comprises a lens body and a primary echo transmitter provided behind the lens body. Further, in order to reduce an electromagnetic wave reflection from the surface of the lens body, a matching layer may be provided on the surface of the lens body. In practice, the lens body as well as the matching layer may be formed by dielectric ceramics and thermoplastic resins.
However, with the above-described conventional lens antenna, a high temperature condition will cause the surface of the lens body or the matching layer to become oxidized and thus deteriorated, so that the lens body will be repeatedly exposed to an undesired thermal expansion or shrinkage. As a result, a stress remaining within the lens body will be applied to the surface thereof or to the matching layer, resulting in cracks occurring in the lens body or the matching layer.
Whenever such cracks occur, not only will the appearance of the lens antenna become worse, but also moisture will invade into the lens antenna, bringing about an undesired change in the lens characteristic. As a result, it will be difficult for the lens antenna to provide a desired gain.